class Solution {
private:
vector<vector<string>> result;
void dfs(int n, int row, vector<string> &chessboard) {
    if (row == n) {
        result.push_back(chessboard);
        return ;
    }
    for (int col = 0; col < n; col++) {
        if (isValid(row, col, chessboard, n)) {
            chessboard[row][col] = 'Q';
            dfs(n, row + 1, chessboard);
            chessboard[row][col] = '.';
        }
    }
    return ;
}
bool isValid(int row, int col, vector<string> &chessboard, int n) {
    for (int i = 0; i < row; i++) {
        if (chessboard[i][col] == 'Q') return false;
    }
    for (int i = row - 1, j = col - 1; i >= 0 && j >= 0; i--, j--) {
        if (chessboard[i][j] == 'Q') return false;
    }
    for (int i = row - 1, j = col + 1; i >= 0 && j < n; i--, j++) {
        if (chessboard[i][j] == 'Q') return false;
    }
    return true;
}
public:
    vector<vector<string>> solveNQueens(int n) {
        result.clear();
        vector<string> chessboard(n, string(n, '.'));
        dfs(n, 0, chessboard);
        return result;
    }
};
